龙门山后山断裂中段汶川-茂县断裂的晚第四纪活动性分析

摘要

龙门山断裂带位于青藏高原东缘与四川盆地交界处,构造位置非常重要,对中国西南地区地形地貌、地质构造以及地震的发生等影响深远.通过卫星影像解译以及野外调查,初步分析认为岷江在汶川-茂县断裂区段主要发育5级阶地.结合前人研究获得的阶地年龄,文中将T1-T5阶地年龄分别限定在3~10ka、约20ka、40~50ka、60ka和80ka.另外,通过对区域内断裂沿线的阶地、冲沟等地质地貌的野外调查,结合卫星影像解译和差分GPS测量等工作,认为汶川-茂县断裂在T3阶地(40～50ka BP)形成后、T2阶地(约20ka BP)形成前有过活动,而在T2阶地形成以来不再活动.结合中央断裂、前山断裂均为全新世活动断裂的认识,推测龙门山断裂带的活动已转移到更靠近盆地的中央断裂和前山断裂上.%The Longmenshan fault zone is located in eastern margin of Tibetan plateau and bounded on the east by Sichuan Basin,and tectonically the location is very important.It has a deep impact on the topography,geomorphology,geological structure and seismicity of southwestern China.It is primarily composed of multiple parallel thrust faults,namely,from northwest to southeast,the back-range,the central,the front-range and the piedmont hidden faults,respectively.The MS8.0 Wenchuan earthquake of 12th May 2008 ruptured the central and the front-range faults.But the earthquake didn`t rupture the back-range fault.This shows that these two faults are both active in Holocene.But until now,we don`t know exactly the activity of the back-range fault.The back-range fault consists of the Pingwu-Qingchuan Fault,the Wenchuan-Maoxian Fault and the Gengda-Longdong Fault.Through satellite image(Google Earth)interpretation,combining with field investigation,we preliminarily found out that five steps of alluvial platforms or terraces have been developed in Minjiang region along the Wenchuan-Maoxian Fault.T1 and T2 terraces are more continuous than T3,T4 and T5 terraces.Combining with the previous work,we discuss the formation ages of the terraces and conclude,analyze and summarize the existing researches about the terraces of Minjiang River.We constrain the ages of T1,T2,T3,T4 and T5 surfaces to 3～10ka BP,～20ka BP,40～50ka BP,60ka BP and 80ka BP,respectively.Combining with geomorphologic structural interpretation,measurements of the cross sections of the terraces by differential GPS and detailed site visits including terraces,gullies and other geologic landforms along the fault,we have reason to consider that the Wenchuan-Maoxian Fault was active between the formation age of T3 and T2 terrace,but inactive since T2 terrace formed.Its latest active period should be the middle and late time of late Pleistocene,and there is no activity since the Holocene.Combining with the knowledge that the central and the front-range faults are both Quaternary active faults,the activity of Longmenshan fault zone should have shifted to the central and the front-range faults which are closer to the basin,this indicates that the Longmenshan thrust belt fits the "Piggyback Type" to some extent.